Many imaging apparatuses equipped with a backlighting compensation means have been developed in recent years. One such imaging apparatus is disclosed in Japanese Patent Laid-Open Publication (tokkai) H4-340875 (1992-340875) and described below.
FIG. 23 is a block diagram of the imaging apparatus disclosed in Japanese Patent Laid-Open Publication (tokkai) H4-340875 (1992-340875). Referring to FIG. 23, this imaging apparatus comprises a lens 1; aperture mechanism 2; imaging element 3; a preamplifier 4 for amplifying the output from the imaging element 3 to an acceptable level; an integration circuit 5; an aperture control circuit 6; a process circuit 7 comprising a gamma correction circuit and white balance circuit; an automatic gain control (AGC) circuit 8; an integration circuit 9; an AGC control circuit 10 for generating a signal controlling the gain of the AGC circuit 8; an A/D converter 11 for analog-digital conversion of the image signal output from the AGC circuit 8; an image segmentation circuit 12 for segmenting the A/D-converted signal into plural segments; a multiplier 13 for calculating an evaluation value for the brightness of each signal segment in the segmented signal output by the image segmentation circuit 12; a frequency distribution calculator 14 for obtaining the frequency distribution of the signal segment brightness levels by counting the number of signal segments at each brightness level; an interface circuit 15 for inputting the output of the multiplier 13 and the output of the frequency distribution calculator 14 to the microprocessor 16; a D/A converter 17 for converting the digital signal output of the microprocessor 16 to an analog signal; a control signal generator 18 for generating a control signal according to the output from the D/A converter 17; a gain control circuit 19 for controlling the image signal gain based on the control signal output from the control signal generator 18; a camera signal processing circuit 20; and a signal output terminal 21.
The imaging apparatus thus comprised operates as follows.
The amount of light passing the lens 1 is limited by the aperture mechanism 2, converted to an electrical signal by the imaging element 3, and then amplified by the preamplifier 4. The output from the preamplifier 4 is integrated by the integration circuit 5, thus producing a dc signal corresponding to the output signal level from the preamplifier 4. This dc signal is supplied to the aperture control circuit 6.
The aperture control circuit 6 then compares the dc signal level input from the integration circuit 5 with a reference voltage to generate and output a control signal causing the aperture mechanism 2 to operate such that the output signal level of the preamplifier 4 is constant.
The output from the preamplifier 4 is also supplied to the process circuit 7 for gamma correction and white balance control, and is then output to the AGC circuit 8. The output of the AGC circuit 8 is integrated by another integration circuit 9, thus producing a dc signal based on the output signal level of the AGC circuit 8. This dc signal is then compared by the AGC control circuit 10 with a reference voltage to generate an AGC control signal used to control the AGC circuit 8 to output at a constant output signal level.
The output from the AGC circuit 8 is then converted to a digital signal by the A/D converter 11, and the resulting digital signal is segmented by the image segmentation circuit 12 into plural signal segments corresponding to specific image areas on screen. The multiplier 13 then detects the average luminance distribution of the video signal in each image segment as the exposure value of each segment, and the frequency distribution calculator 14 obtains the luminance distribution in each segment. The microprocessor 16 then determines the correlation between the image center and the other image segments, and defines as the main subject area the area with a correlation to the image center, and defines the other image areas as secondary subject areas. Based on the ratio of main subject areas to secondary subject areas, the microprocessor 16 then detects backlighting and strong normal lighting to control the image signal gain according to the backlighting-normal lighting ratio.
When controlling the image signal gain, compensation is applied so that gain is greater in the low luminance areas than in the high luminance level areas of the video signal. As a result of this process, the gradation characteristics of dark image areas are compensated so that an image signal with contrast is output from the gain control circuit 19. This signal is then variously processed by the signal processing circuit 20 to produce the video signal output from the signal output terminal 21.
It should be noted that the conventional technology described above detects the rate of the backlighting and strong normal lighting and controls the video signal gain according to this rate. When controlling the video signal gain the gain for low luminance level image areas is made higher than the gain for high luminance level areas in the image signal. It is therefore possible to provide gradation correction to dark image areas but at a cost of increasing the signal-to-noise (S/N) ratio in low luminance parts of the image signal.
An imaging apparatus comprising a gradation compensation circuit for improving the S/N ratio of low luminance image areas has been previously disclosed in previous filings by the inventors, specifically in U.S. Ser. No. 08/201,426 (Feb. 24, 1994) and EP Appln. No. 94 102 684.1 (Feb. 23, 1994). The problem with said apparatus, however, is insufficient improvement of the S/N ratio.